This invention relates to wastewater treatment systems, and more particularly to wastewater treatment systems for treating liquid from which solids have been removed by gravitational separation, such as the effluent from a septic tank, prior to on-site disposal.
Where soil conditions will not permit use of a seepage field system for the direct disposal of wastewater from which solids have been gravitationally removed, some type of preliminary treatment system must be used prior to the on-site disposal of the wastewater. Such a preliminary treatment system often includes a particulate media filter because a particulate media filter is economical to install and requires minimal maintenance compared to other preliminary treatment methods.
The particulate media filter treats wastewater, typically septic tank effluent, by removing additional particulate matter and biodegradable material by a combination of mechanical filtration and aerobic biological activity. The filter media is a particulate material such as sand which serves as a support conducive to the growth and maintenance of aerobic organisms. Septic tank effluent, deposited in or on the filter media, is allowed to flow by gravity down through the media. As the effluent flows through and around the media particles, the biological activity of the aerobic organisms supported by each particle degrades organic material present in the effluent. In addition to the biological activity, larger solid materials present in the effluent tend to remain in the media interstices as the effluent passes through the filter.
The most common problem associated with a particulate media filter is the undesirable growth of anaerobic biological organisms. Such anaerobic growth tends to accumulate in the distribution system of pipes feeding the septic tank effluent to the filter, as well as in the filter itself. The undesired biological growth can clog the orifices in the distribution pipes in the absence of frequent maintenance, inhibiting the even distribution of effluent through the filter and interfering with the efficiency of the filter. Biological overloading of the portion of the filter not having clogged orifices accelerates anaerobic growth in and on the filter media. With time, the anaerobic organisms, which break down organic matter at a rate many times slower than aerobic organisms do, crowd out the aerobic ones and the filter can no longer sufficiently treat the daily wastewater flow for which it is designed.
Despite the persistence of these problems, prior wastewater filtering systems have been unable to effectively inhibit such undesirable biological growth. Because of the aforementioned distribution pipe clogging problem, in a typical filter the orificed pipes for distributing effluent over the filter are normally selected on the basis of the largest diameter pipes that are economically feasible. Thus, orificed distribution pipes having a nominal 11/4 to 2 inch diameter are commonly used in such filters, in the belief that the large cross-sectional area of the pipes will minimize clogging and maximize the uniformity of effluent distribution. Nevertheless, such filters regularly lose their efficiency or fail because of clogging biological growth in the pipes and filter beds.
Accepted standards for large-diameter pipes in sanitary sewers dictate a design velocity of not less than 0.60 m/sec (2 feet per second) or generally greater than 3.5 m/sec (10 feet per second) at peak flow. The minimum velocity requirement is necessary to prevent the deposition of raw sewage solids, as explained in Gravity Sanitary Sewer Design and Construction, No. 60 ASCE Manuals and Reports on Engineering Practice and No. FD-5 WPCF Manual of Practice 122 (1982). However, because such solids have been gravitationally removed from septic tank effluent, such reason for a minimum velocity in a wastewater treatment filter system for septic tank effluent is inapplicable. Accordingly, no reason has existed in the past for filter designers to consider reducing the cross-sectional area of the filter distribution pipes to increase effluent velocities, especially in view of the belief that such reduction would contribute to clogging.
What is still needed, therefore, is a septic tank effluent treatment system with an easily maintained particulate media filter that minimizes the undesirable biological growth in the system conduits and particulate media while maximizing filter efficiency through even distribution of effluent in the filter.
Morrison, et al., U.S. Pat. No. 2,605,901; Hungerford, et al., U.S. Pat. No. 2,092,716; and Jewell, U.S. Pat. No. 649,411 disclose the use of screens between the layers of a filter. None of these screens, however, is properly located or has a mesh size small enough to aid in the even distribution of gravity-flow effluent in a particulate media filter.
Kuh, et al., U.S. Pat. No. 4,681,677 discloses a pressure differential sensor for use with a drinking water processor device. When the pressure differential between the inlet and outlet of a filter is large enough, indicating that the filter has failed and must be replaced, a shut-off valve or bypass device is activated to interrupt or intercept the effluent flow of water. Such a system, however, does not address the problem of clogging of the distribution system of a wastewater particulate media filter, nor enable correction of the problem prior to filter failure.
Duden, U.S. Pat. No. 1,784,893 discloses water treatment apparatus that includes relatively large intake nozzles imbedded a slight distance below the top level of a filter bed when no water is passing therethrough. The relatively large mouth of each intake nozzle is covered by a metal plate having a large number of cylindrical orifices of such size as to prevent the passage into the nozzles of any filter media as water flows through the bed in an upward direction. However, there is no suggestion as to how clogging of the imbedded orifices with filter media particles can be avoided.
Walker, U.S. Pat. No. 2,439,764 discloses water treatment apparatus having distributor arms provided with a plurality of orifices and rotating above a filter bed. An aerator plate, adjustably carried by a bracket, is located in front of each orifice. However, the aeration function precludes imbedding of the orifices in the filter media, and therefore does not present a problem of orifice clogging with filter media particles.